1. Field of the Invention
Example embodiments generally relate to a fixing device and an image forming apparatus, and more particularly, to a fixing device for fixing a toner image on a recording medium and an image forming apparatus including the fixing device.
2. Description of the Related Art
Related-art image forming apparatuses, such as copiers, facsimile machines, printers, or multifunction printers having at least one of copying, printing, scanning, and facsimile functions, typically form an image on a recording medium according to image data. Thus, for example, a charger uniformly charges a surface of an image carrier; an optical writer emits a light beam onto the charged surface of the image carrier to form an electrostatic latent image on the image carrier according to the image data; a development device supplies toner to the electrostatic latent image formed on the image carrier to render the electrostatic latent image visible as a toner image; the toner image is directly transferred from the image carrier onto a recording medium or is indirectly transferred from the image carrier onto a recording medium via an intermediate transfer member; a cleaner then collects residual toner not transferred and remaining on the surface of the image carrier after the toner image is transferred from the image carrier onto the recording medium; finally, a fixing device applies heat and pressure to the recording medium bearing the toner image to fix the toner image on the recording medium, thus forming the image on the recording medium.
Such image forming apparatuses may employ a fixing device incorporating a fixing belt heated by electromagnetic induction to shorten a warm-up time required to warm up the fixing belt to a predetermined fixing temperature and a first print time required to output a recording medium bearing a fixed toner image after the image forming apparatus receives a print job.
For example, the fixing belt formed into a loop contacts an opposed pressing roller to form a fixing nip therebetween through which a recording medium bearing an unfixed toner image is conveyed. As a recording medium bearing an unfixed toner image is conveyed through the fixing nip, the fixing belt heated by electromagnetic induction by an exciting coil and a temperature sensitive magnetic member, together with the pressing roller, applies heat and pressure to the recording medium, thus melting and fixing the toner image on the recording medium. The temperature sensitive magnetic member separatably contacts the inner circumferential surface of the fixing belt and is disposed opposite the exciting coil via the fixing belt. As the temperature sensitive magnetic member receives a magnetic flux from the exciting coil, it heats the fixing belt by electromagnetic induction. Accordingly, the temperature sensitive magnetic member is isolated from the inner circumferential surface of the fixing belt with a predetermined gap therebetween until the fixing belt is heated to the predetermined fixing temperature, so that the temperature sensitive magnetic member does not draw heat from the fixing belt, thus facilitating quick heating of the fixing belt. After that, the temperature sensitive magnetic member comes into contact with the fixing belt. However, even while the temperature sensitive magnetic member is isolated from the fixing belt before the fixing belt reaches the predetermined fixing temperature, the temperature sensitive magnetic member may be constantly heated by the magnetic flux from the exciting coil, degrading heating efficiency for heating the fixing belt.
Another fixing device incorporates a fixing roller instead of the fixing belt and a conductive member instead of the temperature sensitive magnetic member. The conductive member situated inside the fixing roller is rotatable in the circumferential direction of the fixing roller and thus movable between the opposed position where the conductive member is disposed opposite the exciting coil via the fixing roller and the non-opposed position where the conductive member is not disposed opposite the exciting coil. The conductive member is at the non-opposed position until the fixing roller is heated to the predetermined fixing temperature. After that, the conductive member moves to the opposed position. However, the fixing roller incorporates a heat generation layer heated by electromagnetic induction by a magnetic flux from the exciting coil and a temperature sensitive magnetic layer having a predetermined Curie temperature that prevents overheating of the fixing roller. Since the heat generation layer is combined with the temperature sensitive magnetic layer, the temperature sensitive magnetic layer may draw heat from the heat generation layer, hindering quick heating of the fixing roller.